Scanned imaging apparatus with switched feeds

ABSTRACT

A display apparatus includes a scanning assembly that scans about two or more axes, typically in a raster pattern. A plurality of light sources emit light from spaced apart locations toward the scanning assembly such that the scanning assembly simultaneously scans more than one of the beams. The light sources are positioned such that their beams each illuminate discrete regions of the image field that are substantially nonoverlapping with respect to the other discrete regions. The image is thus formed from a set of “tiles”. By activating a first light source during a forward sweep of the mirror and activating a second light source during a reverse sweep of the mirror, two halves a common line can be written during a single sweep of the mirror. Shifting the position of the sources such that the two halves are aligned reduces raster pinch. In alternative embodiments, the same approach is used for imaging. Also, various approaches to controlling the frequency responses of the various scanners are described, including active control of MEMs scanners and passive frequency tuning.

TECHNICAL FIELD

[0001] The present invention relates to scanned light devices and, moreparticularly, to scanned light beam imaging devices for viewing orcollecting images.

BACKGROUND OF THE INVENTION

[0002] A variety of techniques are available for providing visualdisplays of graphical or video images to a user. In many applicationscathode ray tube type displays (CRTs), such as televisions and computermonitors produce images for viewing. Such devices suffer from severallimitations. For example, CRTs are bulky and consume substantial amountsof power, making them undesirable for portable or head-mountedapplications.

[0003] Matrix addressable displays, such as liquid crystal displays andfield emission displays, may be less bulky and consume less power.However, typical matrix addressable displays utilize screens that areseveral inches across. Such screens have limited use in head mountedapplications or in applications where the display is intended to occupyonly a small portion of a user's field of view. Such displays have beenreduced in size. at the cost of increasingly difficult processing andlimited resolution or brightness. Also, improving resolution of suchdisplays typically requires a significant increase in complexity.

[0004] One approach to overcoming many limitations of conventionaldisplays is a scanned beam display, such as that described in U.S. Pat.No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, whichis incorporated herein by reference. As shown diagrammatically in FIG.1, in one embodiment of a scanned beam display 40, a scanning source 42outputs a scanned beam of light that is coupled to a viewer's eye 44 bya beam combiner 46. In some scanned displays, the scanning source 42includes a scanner, such as scanning mirror or acousto-optic scanner,that scans a modulated light beam onto a viewer's retina. In otherembodiments, the scanning source may include one or more light emittersthat are rotated through an angular sweep.

[0005] The scanned light enters the eye 44 through the viewer's pupil 48and is imaged onto the retina 59 by the cornea. In response to thescanned light the viewer perceives an image. In another embodiment, thescanned source 42 scans the modulated light beam onto a screen that theviewer observes. One example of such a scanner suitable for either typeof display is described in U.S. Pat. No. 5,557,444 to Melville et al.,entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, whichis incorporated herein by reference.

[0006] Sometimes such displays are used for partial or augmented viewapplications. In such applications, a portion of the display ispositioned in the user's field of view and presents an image thatoccupies a region 43 of the user's field of view 45, as shown in FIG.2A. The user can thus see both a displayed virtual image 47 andbackground information 49. If the background light is occluded, theviewer perceives only the virtual image 47, as shown in FIG. 2B.

[0007] One difficulty that may arise with such displays is raster pinch,as will now be explained with reference to FIGS. 3-5. As showndiagrammatically in FIG. 3, the scanning source 42 includes an opticalsource 50 that emits a beam 52 of modulated light. In this embodiment,the optical source 50 is an optical fiber that is driven by one or morelight emitters, such as laser diodes (not shown). A lens 53 gathers andfocuses the beam 52 so that the beam 52 strikes a turning mirror 54 andis directed toward a horizontal scanner 56. The horizontal scanner 56 isa mechanically resonant scanner that scans the beam 52 periodically in asinusoidal fashion. The horizontally scanned beam then travels to avertical scanner 58 that scans periodically to sweep the horizontallyscanned beam vertically. For each angle of the beam 52 from the scanners58, an exit pupil expander 62 converts the beam 52 into a set of beams63. Eye coupling optics 60 collect the beams 63 and form a set of exitpupils 65. The exit pupils 65 together act as an expanded exit pupil forviewing by a viewer's eye 64. One such expander is described in U.S.Pat. No. 5,701,132 of Kollin et al., entitled VIRTUAL RETINAL DISPLAYWITH EXPANDED EXIT PUPIL, which is incorporated herein by reference. Oneskilled in the art will recognize that, for differing applications, theexit pupil expander 62 may be omitted, may be replaced or supplementedby an eye tracking system, or may have a variety of structures,including diffractive or refractive designs. For example, the exit pupilexpander 62 may be a planar or curved structure and may create anynumber or pattern of output beams in a variety of patterns. Also,although only three exit pupils are shown in FIG. 3, the number ofpupils may be almost any number. For example, in some applications a 15by 15 array may be suitable.

[0008] Returning to the description of scanning, as the beam scansthrough each successive location in the beam expander 62, the beam colorand intensity is modulated in a fashion to be described below to form arespective pixel of an image. By properly controlling the color andintensity of the beam for each pixel location, the display 40 canproduce the desired image.

[0009] Simplified versions of the respective waveforms of the verticaland horizontal scanners are shown in FIG. 4. In the plane 66 (FIG. 3),the beam traces the pattern 68 shown in FIG. 5. Though FIG. 5 shows onlyeleven lines of image, one skilled in the art will recognize that thenumber of lines in an actual display will typically be much larger thaneleven. As can be seen by comparing the actual scan pattern 68 to adesired raster scan pattern 69, the actual scanned beam 68 is “pinched”at the outer edges of the beam expander 62. That is, in successiveforward and reverse sweeps of the beam, the pixels near the edge of thescan pattern are unevenly spaced. This uneven spacing can cause thepixels to can cause the pixels to overlap or can leave a gap betweenadjacent rows of pixels. Moreover, because the image information istypically provided as an array of data, where each location in the arraycorresponds to a respective position in the ideal raster pattern 69, thedisplaced pixel locations can cause image distortion.

[0010] For a given refresh rate and a given wavelength, the number ofpixels per line is determined in the structure of FIG. 3 by the mirrorscan angle θ and mirror dimension D perpendicular to the axis ofrotation. For high resolution, it is therefor desirable to have a largescan angle θ and a large mirror. However, larger mirrors and scan anglestypically correspond to lower resonant frequencies. A lower resonantfrequency provides fewer lines of display for a given period.Consequently, a large mirror and larger scan angle may produceunacceptable refresh rates.

SUMMARY OF THE INVENTION

[0011] A display includes a primary scanning mechanism thatsimultaneously scans a plurality of beams of light both horizontally andvertically along substantially continuous scan paths where each beamdefines a discrete “tile” of an image. In the preferred embodiment, thescanning mechanism includes a mirror that pivots to sweep the beamshorizontally.

[0012] Optical sources are aligned to provide the beams of light to thescanning mechanism from respective input angles. The input angles areselected such that the scanning mechanism sweeps each beam of lightacross a respective distinct region of an image field. Because therespective regions are substantially non-overlapping, each beam of lightgenerates a substantially spatially distinct region of the image. Therespective regions are immediately adjacent or may overlap, so that thespatially distinct regions are “tiled” to form a contiguous image.Because movement of the mirror produces movement of all of the beams,the display produces each of the spatially separate regionssimultaneously. As described above, the scan angle θ and the mirrordimensions determine the number of pixels drawn for each beam. The totalnumber of pixels in a line can thus substantially equal the number ofpixels for each beam multiplied by the number of beams.

[0013] In one embodiment, the scanning mechanism scans in a generallyraster pattern with a horizontal component and a vertical component. Amechanically resonant scanner produces the horizontal component byscanning the beam sinusoidally. A non-resonant or semi-resonant scannertypically scans the beam vertically with a substantially constantangular speed.

[0014] In one embodiment, the scanning mechanism includes a biaxialmicroelectromechanical (MEMs) scanner. The biaxial scanner uses a singlemirror to provide both horizontal and vertical movement of each of thebeams. In one embodiment, the display includes a buffer that stores dataand outputs the stored data to each of the optical sources. A correctionmultiplier provides correction data that adjusts the drive signals tothe optical sources in response to the stored data. The adjusted drivesignals compensate for variations in output intensity caused by patterndependent heating.

[0015] In another embodiment, an imager acquires images in tiles byutilizing two or more separate detector and optical source pairs. Oneembodiment of the imager includes LEDs or lasers as the optical sources,where each of the optical sources is at a respective wavelength. Thescanning assembly simultaneously directs light from each of the opticalsources to respective regions of an image field. For each location inthe image field, each of the detectors selectively detects light at thewavelength, polarization, or other characteristic of its correspondingsource, according to the reflectivity of the respective location. Thedetectors output electrical signals to decoding electronics that storedata representative of the image field.

[0016] In one embodiment, the imager includes a plurality ofdetector/optical source pairs at each of red, green, and blue wavelengthbands. Each pair operates at a respective wavelength within its band.For example, a first of the red pairs operates at a first red wavelengthand a second of the red pairs operates at a second red wavelengthdifferent from the first.

[0017] In one embodiment, a pair of optical sources alternately feed asingle scanner from different angles. During forward sweeps of thescanner, a first of the sources emits light modulated according to onehalf of a line. During the return sweep, the second source emits lightmodulated according to the second half of the line. Because the secondsweep is in the opposite direction from the first, data corresponding tothe second half of the line is reversed before being applied to thesecond source so that light from the second source is modulated to writethe second half of the line in reverse.

[0018] In one embodiment of the alternate feeding approach, a singlelight emitter feeds an input fiber that is selectively coupled to one oftwo separate fibers by an optical switch. During forward sweeps, theoptical switch couples the input fiber to a first of the separate fibersso that the first separate fiber forms the first optical source. Duringreverse sweep, the optical switch feeds the second separate fiber sothat the second separate fiber forms the second source. This embodimentthus allows a single light emitter to provide light for both opticalsources.

[0019] The alternate feeding approach can be expanded to write more thanjust two tiles. In one approach, the input fiber is coupled to fourfibers by a set of optical switches, where each fiber feeds the scanningassembly from a respective angle. The switches are activated accordingto the direction of the sweep and according to the tracked location ofthe user's vision. For example, when the user looks at the top half ofthe image, a first fiber, aligned to produce an image in the upper lefttile feeds the scanner during the forward sweeps. A second fiber,aligned to produce an upper right tile feeds the scanner during reversesweeps. When the user looks at the lower half of the image, a thirdfiber, aligned to produce the lower left tile, feeds scanner duringforward sweeps. A fourth fiber, aligned to produce the lower right tile,feeds the scanner during reverse sweeps.

BRIEF DESCRIPTION OF THE FIGURES

[0020]FIG. 1 is a diagrammatic representation of a display aligned to aviewer's eye.

[0021]FIG. 2A is a combined image perceived by a user resulting from thecombination of light from an image source and light from a background.

[0022]FIG. 2B is an image perceived by a user from the display of FIG. 1where the background light is occluded.

[0023]FIG. 3 is a diagrammatic representation of a scanner and a user'seye showing bidirectional scanning of a beam and coupling to theviewer's eye.

[0024]FIG. 4 is a signal-timing diagram of a scan pattern scanner in thescanning assembly of FIG. 3.

[0025]FIG. 5 is a signal position diagram showing the path followed bythe scanned beam in response to the signals of FIG. 4, as compared to adesired raster scan path.

[0026]FIG. 6 is a diagrammatic representation of a display according tothe one embodiment invention including dual light beams.

[0027]FIG. 7 is an isometric view of a head-mounted scanner including atether.

[0028]FIG. 8 is a diagrammatic representation of a scanning assemblywithin the scanning display of FIG. 6, including a correction mirror.

[0029]FIG. 9 is an isometric view of a horizontal scanner and a verticalscanner suitable for use in the scanning assembly of FIG. 8.

[0030]FIG. 10 is a diagrammatic representation of scanning with twoinput beams, showing slightly overlapped tiles.

[0031]FIG. 11 is a top plan view of a biaxial scanner showing four feedsat spatially separated locations.

[0032]FIG. 12 is a diagrammatic representation of four tiles produces bythe four feed scanner of FIG. 11.

[0033]FIG. 13 is a schematic of a system for driving the four separatefeeds of FIG. 11, including four separate buffers.

[0034]FIG. 14 is a signal-timing diagram comparing a ramp signal with adesired signal for driving the vertical scanner.

[0035]FIG. 15 is a signal timing diagram showing positioning error andcorrection for the vertical scanning position.

[0036]FIG. 16 is a side cross sectional view of a piezoelectriccorrection scanner.

[0037]FIG. 17A is a top plan view of a microelectromechanical (MEMs)correction scanner.

[0038]FIG. 17B is a side cross-sectional view of the MEMs correctionscanner of FIG. 17A showing capacitive plates and their alignment to thescanning mirror.

[0039]FIG. 18 shows corrected scan position using a sinusoidally drivenscanner through 90% of the overall scan.

[0040]FIG. 19 shows an alternative embodiment of a reduced error scannerwhere scan correction is realized by adding a vertical component to thehorizontal mirror.

[0041]FIG. 20 is a position diagram showing the scan path of a beamdeflected by the scanner of FIG. 19.

[0042]FIG. 21 is a diagrammatic view of a scanning system, including abiaxial microelectromechanical (MEMs) scanner and a MEMs correctionscanner.

[0043]FIG. 22 is a diagrammatic view of a correction scanner that shiftsan input beam by shifting the position or angle of the input fiber.

[0044]FIG. 23 is a diagrammatic view of a correction scanner thatincludes an electro-optic crystal that shifts the input beam in responseto an electrical signal.

[0045]FIG. 24 is a diagrammatic view of an imager that acquires externallight from a target object.

[0046]FIG. 25 is a diagrammatic view of an alternative embodiment of theimager of FIG. 24 that also projects a visible image.

[0047]FIG. 26 is a signal timing diagram showing deviation of asinusoidal scan position versus time from the position of a linear scan.

[0048]FIG. 27 is a diagram showing diagrammatically how a linear set ofcounts can map to scan position for a sinusoidally scan.

[0049]FIG. 28 is a system block diagram showing handling of data tostore data in a memory matrix while compensating for nonlinear scanspeed of the resonant mirror.

[0050]FIG. 29 is a block diagram of a first system for generating anoutput clock to retrieve data from a memory matrix while compensatingfor nonlinear scan speed of the resonant mirror.

[0051]FIG. 30 is a block diagram of an alternative embodiment of theapparatus of FIG. 29 including pre-distortion.

[0052]FIG. 31 is a detail block diagram of a clock generation portion ofthe block diagram of FIG. 29.

[0053]FIG. 32 is a representation of a data structure showing datapredistorted to compensate for vertical optical distortion.

[0054]FIG. 33 is a top plan view of a MEMs scanner including structuresfor electronically controlling the center of mass of each mirror half

[0055]FIG. 34 is a top plan view of the MEMs scanner of FIG. 32 showingflexing of protrusions in response to an applied voltage.

[0056]FIG. 35 is a top plan view of a MEMs scanner including combstructures for laterally shifting the center of mass of each mirrorhalf.

[0057]FIG. 36 is a side cross sectional view of a packaged scannerincluding electrically controlled outgassing nodules.

[0058]FIG. 37 is a top plan view of a MEMs mirror including selectivelyremovable tabs for frequency tuning.

[0059]FIG. 38 is a diagrammatic view of a four source display showingoverlap of scanning fields with optical sources.

[0060]FIG. 39 is a diagrammatic view of a four source display with smallturning mirrors and offset optical sources.

[0061]FIG. 40 is a diagrammatic view of the display of FIG. 39 showingbeam paths with the small turning mirrors and a common curved mirror.

[0062]FIG. 41 is a diagrammatic view of a single emitter displayincluding switched optical fibers each feeding a separate tile.

[0063]FIG. 42 is a diagrammatic view of a display including fourseparate fibers feeding a scanner through a set of optical switches inresponse to a detected gaze direction to produce four separate tiles.

DETAILED DESCRIPTION OF THE INVENTION

[0064] As shown in FIG. 6, a scanned beam display 70 according to oneembodiment of the invention is positioned for viewing by a viewer's eye72. While the display 70 is presented herein is scanning light into theeye 72, the structures and concepts described herein can also be appliedto other types of displays, such as projection displays that includeviewing screens.

[0065] The display 70 includes four principal portions, each of whichwill be described in greater detail below. First, control electronics 74provide electrical signals that control operation of the display 70 inresponse to an image signal V_(IM) from an image source 76, such as acomputer, television receiver, videocassette player, DVD player, remotesensor, or similar device.

[0066] The second portion of the display 70 is a light source 78 thatoutputs modulated light beams 80, each having a modulation correspondingto information in the image signal V_(IM). The light source 78 mayutilize coherent light emitters, such as laser diodes or microlasers, ormay use non-coherent sources such as light emitting diodes. Also, thelight source 78 may include directly modulated light emitters such asthe light emitting diodes (LEDs) or may include continuous lightemitters indirectly modulated by external modulators, such asacousto-optic modulators.

[0067] The third portion of the display 70 is a scanning assembly 82that scans the modulated beams 80 through two-dimensional scanningpatterns, such as raster patterns. The scanning assembly 82 preferablyincludes a periodically scanning mirror or mirrors as will be describedin greater detail below with reference to FIGS. 3-4, 8, 11, 19-22.

[0068] Lenses 84, 86 positioned on opposite sides of the scanningassembly 82 act as imaging optics that form the fourth portion of thedisplay 70. The lenses 86 are cylindrical graded index (GRIN) lensesthat gather and shape light from the light source 78. Where the lightsource 78 includes optical fibers that feed the lenses 86, the lenses 86may be bonded to or integral to the fibers. Alternatively, other typesof lenses, such as doublets or triplets, may form the lenses 86. Also,other types of optical elements such as diffractive elements may be usedto shape and guide the light. Regardless of the type of element, theoverall optical train may incorporate polarization sensitive materials,chromatic correction, or any other optical technique for controlling theshape, phase or other characteristics of the light.

[0069] The lens 84 is formed from a curved, partially transmissivemirror that shapes and focuses the scanned beams 80 approximately forviewing by the eye 72. After leaving the lens 84, the scanned beams 80enter the eye 72 through a pupil 90 and strike the retina 92. As eachbeam of scanned modulated light strikes the retina 92, the viewerperceives a respective portion of the image as will be described below.

[0070] Because the lens 84 is partially transmissive, the lens 84combines the light from the scanning assembly 82 with the light receivedfrom a background 89 to produce a combined input to the viewer's eye 72.Although the background 89 is presented herein as a “real-world”background, the background light may be occluded or may be produced byanother light source of the same or different type. One skilled in theart will recognize that a variety of other optical elements may replaceor supplement the lenses 84, 86. For example, diffractive elements suchas Fresnel lenses may replace either or both of the lenses 84, 86.Additionally, a beamsplitter and lens may replace the partiallytransmissive mirror structure of the lens 84. Moreover, various otheroptical elements, such as polarizers, color filters, exit pupilexpanders, chromatic correction elements, eye-tracking elements, andbackground masks may also be incorporated for certain applications.

[0071] Although the elements of FIG. 6 are presented diagrammatically,one skilled in the art will recognize that the components are typicallysized and configured for the desired application. For example, where thedisplay 70 is intended as a mobile personal display the components aresized and configured for mounting to a helmet or similar frame as ahead-mounted display 70, as shown in FIG. 7. In this embodiment, a firstportion 171 of the display 70 is mounted to a head-borne frame 174 and asecond portion 176 is carried separately, for example in a hip belt. Theportions 174, 176 are linked by a fiber optic and electronic tether 178that carries optical and electronic signals from the second portion tothe first portion. An example of a fiber-coupled scanner display isfound in U.S. Pat. No. 5,596,339 of Furness et al., entitled VIRTUALRETINAL DISPLAY WITH FIBER OPTIC POINT SOURCE which is incorporatedherein by reference.

[0072] An exemplary embodiment of the scanning assembly 82 will bedescribed next with reference to FIG. 8. The scanning assembly 82includes several components that correspond to the scanning source 42 ofFIG. 3, where components common to the scanning assembly 82 and scanningsource 42 are numbered the same. Additionally, only central rays 55 arepresented for the beams 52 for clarity of presentation.

[0073] In this embodiment, a pair of fibers 50 emit light from the lightsources 78 (not shown) and the lens 84 is represented as a commonrefractive lens rather than as a partially transmissive mirror. Unlikethe scanning source 42 of FIG. 3, the scanning assembly 82 includes anactive correction mirror 100 that can pivot to scan the light beam 80along the vertical axis. As will be explained below, the correctionmirror 100 produces a varying corrective shift along the vertical axisduring each sweep (forward or reverse) of the horizontal scanner 56. Thecorrective shift offsets vertical movement of the beams 80 caused by thevertical scanner 58 to reduce the overall deviation of the scanningpattern from the desired pattern shown in broken lines in FIG. 5.

[0074] Before describing the effects of the correction mirror 100 andthe relative timing of the various signals, exemplary embodiments ofmechanically resonant scanner 200, 220 suitable for use as thehorizontal scanner 56 and vertical scanner 58 will be described withreference to FIG. 9.

[0075] The principal scanning component of the horizontal scanner 200 isa moving mirror 202 mounted to a spring plate 204. The dimensions of themirror 202 and spring plate 204 and the material properties of thespring plate 204 have a high Q with a natural oscillatory (“resonant”)frequency on the order of 1-100 kHz, where the selected resonantfrequency depends upon the application. For VGA quality output with a 60Hz refresh rate and no interlacing, the resonant frequency is preferablyabout 15-20 kHz. As will be described below, the selected resonantfrequency or the achievable resolution may be changed through the use ofa plurality of feeds.

[0076] A ferromagnetic material mounted with the mirror 202 is driven bya pair of electromagnetic coils 206, 208 to provide motive force tomirror 202, thereby initiating and sustaining oscillation. Theferromagnetic material is preferably integral to the spring plate 204and body of the time mirror 202. Drive electronics 218 provideelectrical signals to activate the coils 206, 208, as described above.Responsive to the electrical signals, the coils 206, 208 produceperiodic electromagnetic fields that apply force to the ferromagneticmaterial, thereby causing oscillation of the mirror 202. If thefrequency and phase of the electric signals are properly synchronizedwith the movement of the mirror 202, the mirror 202 oscillates at itsresonant frequency with little power consumption.

[0077] The vertical scanner 220 is structured very similarly to theresonant scanner 200. Like the resonant scanner 201, the verticalscanner 220 includes a mirror 222 driven by a pair of coils 224, 226 inresponse to electrical signals from the drive electronics 218. However,because the rate of oscillation is much lower for vertical scanning, thevertical scanner 220 is typically not resonant. The mirror 222 receiveslight from the horizontal scanner 201 and produces vertical deflectionat about 30-100 Hz. Advantageously, the lower frequency allows themirror 222 to be significantly larger than the mirror 202, therebyreducing constraints on the positioning of the vertical scanner 220. Thedetails of virtual retinal displays and mechanical resonant scanning aredescribed in greater detail in U.S. Pat. No. 5,467,104, of Furness III,et al., entitled VIRTUAL RETINAL DISPLAY which is incorporated herein byreference.

[0078] One skilled in the art will recognize a variety of otherstructures that may scan a light beam through a generally rasterpattern. For example, spinning polygons or galvanometric scanners mayform either or both of the scanners56, 58 in some applications.

[0079] In another embodiment, a bi-axial microelectromechanical (MEMs)scanner may provide the primary scanning. Such scanners are described inU.S. Pat. No. 5,629,790 to Neukermanns et al., entitled MICROMACHINEDTORSIONAL SCANNER, which is incorporated herein by reference. Like thescanning system described above, the horizontal components of the MEMsscanners are typically defined by mechanical resonances of theirrespective structures, as will be described in greater detail below withreference to FIGS. 17A-B and 21. Like the two scanner system describedabove with reference to FIGS. 3 and 8, such biaxial scanners may suffersimilar raster pinch problems due to movement along the slower scan axisduring sweeps along the faster scan axis. Other scanning approaches mayalso apply. For example, acousto-optic scanners, electrooptic scanners,spinning polygons, or some combination of scanning approaches canprovide the scanning function. Some of these approaches may not requirepinch correction.

[0080] Returning to FIGS. 6, 8 and 9, the fibers 50 output light beams80 that are modulated according to the image signal from the driveelectronics 218. At the same time, the drive electronics 218 activatethe coils 206, 208, 224, 226 to oscillate the mirrors 202, 222. Themodulated beams of light strike the oscillating horizontal mirror 202(of the horizontal scanner 56), and are deflected horizontally by anangle corresponding to the instantaneous angle of the mirror 202. Thedeflected beams then strike the vertical mirror 222 (of the verticalscanner 58) and are deflected at a vertical angle corresponding to theinstantaneous angle of the vertical mirror 222. After expansion by thebeam expander 62, the beams 52 pass through the lens 84 to the eye. Aswill also be described below, the modulation of the optical beams issynchronized with the horizontal and vertical scans so that, at eachposition of the mirrors, the beam color and intensity correspond to adesired virtual image. Each beam therefore “draws” a portion of thevirtual image directly upon the user's retina.

[0081] One skilled in the art will recognize that several components ofthe scanning assembly 82 have been omitted from the FIG. 9 for clarityof presentation. For example, the horizontal and vertical scanners 200,220 are typically mounted to a frame. Additionally, lenses and otheroptical components for gathering, shaping, turning, focusing, orcollimating the beams 80 have been omitted. Also, no relay optics areshown between the scanners 200, 220, although these may be desirable insome embodiments. Moreover, the scanner 200 typically includes one ormore turning mirrors that direct the beam such that the beam strikeseach of the mirrors a plurality of times to increase the angular rangeof scanning. Further, in some embodiments, the scanners 200, 220 areoriented such that the beam can strike the scanning mirrors a pluralityof times without a turning mirror.

[0082] Turning to FIGS. 10 and 11, the effect of the plurality of beams80 will now be described. As is visible in FIG. 10, two fibers 50 emitrespective light beams 80. The GRIN lenses 86 gather and focus the beams80 such that the beams 80 become converging beams 80A, 80B that strike acommon scanning mirror 1090.

[0083] For clarity of presentation, the embodiment of FIG. 10 eliminatesthe mirror 84, as is desirable in some applications. Also, theembodiment of FIG. 10 includes a single mirror 1090 that scans biaxiallyinstead of the dual mirror structure of FIG. 9. Such a biaxial structureis described in greater detail below with reference to FIGS. 11, 17A-Band 21. One skilled in the art will recognize that a dual mirror systemmay also be used, though such a system would typically involve a morecomplex set of ray traces and more complex compensation for differingoptical path lengths.

[0084] Also, although the fibers 50 and lenses 84 of FIG. 10 appearpositioned in a common plane with the scanning mirror 1090, in manyapplications, it may be desirable to position the fibers 50 and lenses84 off-axis, as is visible in FIG. 11. Moreover, where four fiber/lenspairs are used, as in FIG. 11, a beam splitter or other optical elementscan allow the fiber/lens pairs to be positioned where they do not blockbeams 80A-D from other fiber/lens pairs. Alternatively, otherapproaches, such as small turning mirrors can permit repositioning ofthe fiber/lens pairs in non-blocking positions with little effect on theimage quality. Such approaches are described in greater detail belowwith reference to FIGS. 11 and 38-40.

[0085] After exiting the lens 86, the first beam 80A strikes thescanning mirror 1090 and is reflected toward an image field 1094. Thesecond beam 80B is also reflected by the scanning mirror 1090 toward theimage field 1094. As shown by the ray tracing of FIG. 10, the horizontalposition of the beams 80A-B in the image field 1094 will be functions ofthe angular deflection from the horizontal scanner 56 and the positionand orientation of the lens 86 and fiber 50.

[0086] At the image field 1092, the first beam 80A illuminates a firstregion 1092 of the image field 1094 and the second beam 80B illuminatesa second region 1096 that is substantially non-overlapping with respectto the first region 1092. To allow a smooth transition between the tworegions 1092, 1096, the two regions 1092, 1096 overlap slightly in asmall overlap region 1098. Thus, although the two regions aresubstantially distinct, the corresponding image portions may be slightly“blended” at the edges, as will be described below with reference toFIGS. 12 and 13.

[0087] While only two beams 80A-B are visible in FIG. 10, more than twofiber/lens pairs can be used and the fiber/lens pairs need not becoplanar. For example, as can be seen in FIG. 11, four separate lenses86 transmit four separate beams 80A-D from four spatially separatedlocations toward the mirror 1090. As shown in FIG. 12, the mirror 1090reflects each of the four beams 80A-D to a respective spatially distinctregion 1202A-D of the image field 1094.

[0088] Thus, the four beams 80A-D each illuminate four separate “tiles”1202A-D that together form an entire image. One skilled in the art willrecognize that more than four tiles may form the image. For example,adding a third set of fiber/lens pairs could produce a 2-by-3 tile imageor a 3-by-2 tile image.

[0089] To produce an actual image, the intensity and color content ofeach of the beams 80A-D is modulated with image information as themirror 1090 sweeps through a periodic pattern, such as a raster pattern.FIG. 13 shows diagrammatically one embodiment where the beams 80A-D canbe modulated in response to an image signal V_(IM) to produce the fourtiles 1202A-D.

[0090] The image signal V_(IM) drives and A/D converter 1302 thatproduces corresponding data to drive a demultiplexer 1304. In responseto the data and a clock signal CK from the controller 74 (FIG. 8), thedemultiplexer 1304 produces four output data streams, where each datastream includes data corresponding to a respective image tile 1202A-D.For example, the demultiplexer 1304 outputs data corresponding to thefirst half of the first line of the image to a first buffer 1306A andthe data corresponding to the second half of the first line to a secondbuffer 1306B. The demultiplexer 1304 then outputs data corresponding tothe second line of the image to the second lines of the first twobuffers 1306A, B. After the first two buffers 1306A, B contain datarepresenting the upper half of the image, the demultiplexer 1304 thenbegins filling third and fourth buffers 1306C, D. Once all of thebuffers 1306A-D are full, an output clock CKOUT clocks datasimultaneously from all of the buffers 1306A-D to respective D/Aconverters 1308A-D. The D/A converters 1308A-D then drive respectivelight sources 78 to produce light that is scanned into the respectiveregions 2102A-D, as described above. The actual timing of the pixeloutput is controlled by the output clock CKOUT, as described below withreference to FIGS. 28-31.

[0091] One skilled in the art will recognize that, although the systemof FIG. 13 is described for four separate regions 1201A-D, a larger orsmaller number of regions may be used. Also, where some overlap of theregions 1202A-D is desired, common data can be stored in more than onebuffer 1202A-D. Because the sets of common data will duplicate somepixels in the overlapping region, the data may be scaled to limit theintensity to the desired level.

[0092] One approach to improving image quality that is helpful in“matching” the image portions 1202A-D to each other will now bedescribed with reference to FIGS. 14 and 15. Because the angle of thebeams 80A-D is determined by the angles of the vertical and horizontalscanner (for the uniaxial, two scanner system) or the horizontal andvertical angles of the single mirror (for the biaxial scanner), theactual vector angle of the beams 80A-D at any point in time can then bedetermined by vector addition. In most cases, the desired verticalportions of the scan patterns will be a “stair step” scan pattern, asshown by the broken line in FIG. 14.

[0093] If the turning mirror 100 (FIG. 8) is disabled, the patterntraced by the ray will be the same as that described above with respectto FIGS. 3-5. As represented by the solid line in FIG. 14, the actualvertical scan portion of the pattern, shown in solid line, will be anapproximate ramp, rather than the desired stair step pattern.

[0094] On approach to providing the stair step pattern would be to drivethe vertical scanner 58 with the stair step voltage. However, becausethe vertical mirror is a physical system and the stair step involvesdiscontinuous motion, the vertical mirror will not follow the drivesignal exactly. Instead, as the vertical mirror attempts to follow thestair step pattern, the vertical mirror will move at a maximum rateindicated largely by the size and weight of the vertical mirror, thematerial properties of the mirror support structure, the peak voltage orcurrent of the driving signal, and electrical properties of the drivingcircuitry. For typical vertical scan mirror size, configuration, scanangle and driving voltage, the vertical scanner 58 is limited tofrequencies on the order of 100 to 3000 Hz. The desired scan pattern hasfrequency components far exceeding this range. Consequently, driving thevertical scanner 58 with a stair step driving signal can produce avertical scan pattern that deviates significantly from the desiredpattern.

[0095] To reduce this problem, the scanning assembly 82 of FIG. 8separates the vertical scan function into two parts. The overallvertical scan is then a combination of a large amplitude ramp functionat about 60 Hz and a small amplitude correction function at twice thehorizontal rate (e.g., about 30 kHz). The vertical scanner 58 canproduce the large amplitude ramp function, because the 60 Hz frequencyis well below the upper frequency limit of typical scanning mirrors.Correction mirrors 100 replace the turning mirrors 100 and provide thesmall amplitude corrections. The correction mirrors 100 operate at amuch higher frequency than the vertical scanner; however, the overallangular swings of the correction mirrors 100 are very small.

[0096] As can be seen from the signal timing diagram of FIG. 15, thecorrection mirror 100 travels from approximately its maximum negativeangle to its maximum positive angle during the time that the horizontalscanner scans from the one edge of the field of view to the oppositeedge (i.e. from time t₁ to t₂ in FIG. 15). The overall correction angle,as shown in FIGS. 14 and 15, is defined by the amount of downward travelof the vertical scan mirror during a single horizontal scan. Thecorrection angle will vary for various configurations of the display;however, the correction angle can be calculated easily.

[0097] For example, for a display where each image region 1202A-D has1280 vertical lines and a total mechanical vertical scan angle of 10degrees, the angular scan range for each line is about 0.008 degrees(10/1280=0.0078125). Assuming the vertical scanner 58 travels thisentire distance during the horizontal scan, an error correction to besupplied by the correction mirror 100 is about plus or minus 0.0039degrees. The angular correction is thus approximately θ/N, where θ isthe vertical scan angle and N is the number of horizontal lines. Thisnumber may be modified in some embodiments. For example, where thehorizontal scanner 56 is a resonant scanner, the correction angle may beslightly different, because the horizontal scanner 56 will use someportion of the scan time to halt and begin travel in the reversedirection, as the scan reaches the edge of the field of view. Thecorrection angle may also be modified to correct for aberrations inoptical elements or optical path length differences. Moreover, thefrequency of the correction scanner 100 may be reduced by half if datais provided only during one half of the horizontal scanner period(“unidirectional scanning”), although raster pinch is typically notproblematic in unidirectional scanning approaches.

[0098] As can be seen from the timing diagrams of FIGS. 14 and 15, thecorrection mirror 100 will translate the beam vertically by about onehalf of one line width at a frequency of twice that of the horizontalscanner 56. For a typical display at SVGA image quality withbi-directional scanning (i.e., data output on both the forward andreverse sweeps of the horizontal scanner 56), the horizontal scanner 56will resonate at about 15 kHz. Thus, for a typical display, thecorrection scanner 100 will pivot by about one-tenth of one degree atabout 30 kHz. One skilled in the art will recognize that, as theresolution of the display increases, the scan rate of the horizontalscanner 56 increases. The scan rate of the correction mirror 100 willincrease accordingly; but, the pivot angle will decrease. For example,for a display having 2560 lines and an overall scan of 10 degrees, thescan rate of the correction mirror 100 will be about 60 kHz with a pivotangle of about 0.002 degrees. One skilled in the art will recognizethat, for higher resolution, the minimum correction mirror size willtypically increase where the spot size is diffraction limited.

[0099]FIG. 16 shows a piezoelectric scanner 110 suitable for thecorrection mirror 100 in some embodiments. The scanner 110 is formedfrom a platform 112 that carries a pair of spaced-apart piezoelectricactuators 114, 116. The correction mirror 100 is a metallized,substantially planar silicon substrate that extends between theactuators 114, 116. The opposite sides of the piezoelectric actuators114, 116 are conductively coated and coupled to a drive amplifier 120such that the voltage across the actuators 114, 116 are opposite. As isknown, piezoelectric materials deform in the presence of electricfields. Consequently, when the drive amplifier 120 outputs a voltage,the actuators 114, 116 apply forces in opposite directions to thecorrection mirror 100, thereby causing the correction mirror 100 topivot. One skilled in the art will recognize that, although thepiezoelectric actuators 114, 116 are presented as having a single set ofelectrodes and a single layer of piezoelectric material, the actuators114, 116 would typically be formed from several layers. Such structuresare used in commercially available piezoelectric devices to producerelatively large deformations.

[0100] A simple signal generator circuit 122, such as a conventionalramp generator circuit, provides the driving signal for the driveamplifier 120 in response to the detected position of the horizontalscanner 56. The principal input to the circuit 122 is a sense signalfrom a sensor coupled to the horizontal scanner 56. The sense signal canbe obtained in a variety of approaches. For example, as described inU.S. Pat. No. 5,648,618 to Neukermanns et al., entitled MICROMACHINEDHINGE HAVING AN INTEGRAL TORSIONAL SENSOR, which is incorporated hereinby reference, torsional movement of a MEMs scanner can produceelectrical outputs corresponding to the position of the scanning mirror.Alternatively, the position of the mirror may be obtained by mountingpiezoelectric sensors to the scanner, as described in U.S. Pat. No.5,694,237 to Melville, entitled POSITION DETECTION OF MECHANICALRESONANT SCANNER MIRROR, which is incorporated herein by reference. Inother alternatives, the position of the beam can be determined byoptically or electrically monitoring the position of the horizontal orvertical scanning mirrors or by monitoring current induced in the mirrordrive coils.

[0101] When the sense signal indicates that the horizontal scanner 56 isat the edge of the field of view, the circuit 122 generates a rampsignal that begins at its negative maximum and reaches its zero crossingpoint when the horizontal scanner reaches the middle of the field ofview. The ramp signal then reaches its maximum value when the horizontalscan reaches the opposite edge of the field of view. The ramp signalreturns to its negative maximum during the interval when the horizontalscan slows to a halt and begins to return sweep. Because the circuit 122can use the sense signal as the basic clock signal for the ramp signal,timing of the ramp signal is inherently synchronized to the horizontalposition of the scan. However, one skilled in the art will recognizethat, for some embodiments, a controlled phase shift of the ramp signalrelative to the sense signal may optimize performance. Where thecorrection mirror 100 is scanned resonantly, as described below withreference to FIG. 18, the ramp signal can be replaced by a sinusoidalsignal, that can be obtained simply be frequency doubling, amplifyingand phase shifting the sense signal.

[0102] The vertical movements of the beams 80A-D induced by thecorrection mirrors 100 offset the movement of the beams 80A-D caused bythe vertical scanner 58, so that the beams 80A-D remain stationary alongthe vertical axis during the horizontal scan. During the time thehorizontal scan is out of the field of view, the beams 80A-D travelvertically in response to the correction mirrors 100 to the nominalpositions of the next horizontal scan.

[0103] As can be seen from the above discussion, the addition of thepiezoelectrically driven correction mirrors 100 can reduce the rasterpinching significantly with a ramp-type of motion. However, in someapplications, it may be undesirable to utilize ramp-type motion. Onealternative embodiment of a scanner 130 that can be used for thecorrection mirror 100 is shown in FIGS. 17A and 17B.

[0104] The scanner 130 is a resonant micorelectromechanical (MEMs)scanner, fabricated similarly to the uniaxial embodiment described inthe Neukermanns '790 patent. Alternatively, the scanner 130 can be amechanically resonant scanner very similar to the horizontal scanner 54of FIG. 9; however, in such a scanner it is preferred that thedimensions and material properties of the plate and mirror be selectedto produce resonance at about 30 kHz, which is twice the resonantfrequency of the horizontal scanner 200. Further, the materials andmounting are preferably selected so that the scanner 130 has a lower Qthan the Q of the horizontal scanner 56. The lower Q allows the scanner130 to operate over a broader range of frequencies, so that the scanner130 can be tuned to an integral multiple of the horizontal scanfrequency.

[0105] The use of the resonant scanner 130 can reduce the complexity ofthe electrical components for driving the scanner 130 and can improvethe scanning efficiency relative to previously described approaches.Resonant scanners tend to have a sinusoidal motion, rather than thedesired ramp-type motion described above. However, if the frequency,phase, and amplitude of the sinusoidal motion are selectedappropriately, the correction mirror 100 can reduce the pinch errorsignificantly. For example, FIG. 18 shows correction of the rastersignal with a sinusoidal motion of the correction mirror where thehorizontal field of view encompasses 90 percent of the overallhorizontal scan angle. One skilled in the art will recognize that theerror in position of the beam can be reduced further if the field ofview is a smaller percentage of the overall horizontal scan angle.Moreover, even further reductions in the scan error can be realized byadding a second correction mirror in the beam path, although this isgenerally undesirable due to the limited improvement versus cost.Another approach to reducing the error is to add one or more higherorder harmonics to the scanner drive signal so that the scanning patternof the resonant correction scanner 130 shifts from a sinusoidal scancloser to a sawtooth wave.

[0106] Another alternative embodiment of a reduced error scanner 140 isshown in FIG. 19 where the scan correction is realized by adding avertical component to a horizontal mirror 141. In this embodiment, thehorizontal scanner 140 is a MEMs scanner having an electrostatic driveto pivot the scan mirror. The horizontal scanner 140 includes an arrayof locations 143 at which small masses 145 may be formed. The masses 145may be deposited metal or other material that is formed in aconventional manner, such as photolithography. Selected ones of themasses 143 are removed to form an asymmetric distribution about acenterline 147 of the mirror 141. The masses 145 provide a component toscan the correction along the vertical axis by pivoting about an axisorthogonal to its primary axis. As can be seen in FIG. 20, the verticalscan frequency is double the horizontal scan frequency, therebyproducing the Lissajous or “bow-tie” overall scan pattern of FIG. 20.The masses 145 may be actively varied (e.g. by laser ablation) to tunethe resonant frequency of the vertical component. This embodiment allowscorrection without an additional mirror, but typically requires matchingthe resonant frequencies of the vibration and the horizontal scanner.

[0107] To maintain matching of the relative resonant frequencies of thehorizontal scanner 56 and the correction scanner 100, the resonantfrequency of either or both scanners 56, 100 may be tuned actively.various frequency control techniques are described below with referenceto FIGS. 33-36. Where the Q of the scanners 56, 100 are sufficiently lowor where the scanners 56, 100 are not resonant, simply varying thedriving frequency may allow the scanning frequency sufficiently tomaintain synchronization without an excessive loss of scanningamplitude.

[0108] As shown in FIG. 21, another embodiment of a scanner 150according to the invention employs a biaxial scanner 152 as theprincipal scan component, along with a correction scanner 154. Thebiaxial scanner 152 is a single mirror device that oscillates about twoorthogonal axes. Design, fabrication and operation of such scanners aredescribed for example in the Neukermanns '790 patent, in Asada, et al,Silicon Micromachined Two-Dimensional Galvano Optical Scanner, IEEETransactions on Magnetics, Vol. 30, No. 6, 4647-4649, November 1994, andin Kiang et al, Micromachined Microscanners for Optical Scanning, SPIEproceedings on Miniaturized Systems with Micro-Optics and MicromachinesII, Vol. 3008, February 1997, pp. 82-90 each of which is incorporatedherein by reference. The bi-axial scanner 152 includes integral sensors156 that provide electrical feedback of the mirror position to terminals158, as is described in the Neukermanns '618 patent.

[0109] The correction scanner 154 is preferably a MEMs scanner such asthat described above with reference to FIGS. 17A-B, although other typesof scanners, such as piezoelectric scanners may also be within the scopeof the invention. As described above, the correction mirror 154 can scansinusoidally to remove a significant portion of the scan error; or, thecorrection mirror can scan in a ramp pattern for more precise errorcorrection.

[0110] Light from the light source 78 strikes the correction mirror 154and is deflected by a correction angle as described above. The lightthen strikes the biaxial scanner 152 and is scanned horizontally andvertically to approximate a raster pattern, as described above withreference to FIGS. 3-5. Although this embodiment places the correctionmirror 154 before the biaxial scanner 152, the order may be reversed insome applications, although it may be necessary to adjust thecharacteristics of the correction mirror 154 or biaxial scanner 152 toaccommodate the scan paths.

[0111] Another embodiment of a display according to the invention, shownin FIG. 23, eliminates the correction mirror 100 by physically shiftingthe input beam laterally relative to the input of an optical system 500.In the embodiment of FIG. 23, a piezoelectric driver 502 positionedbetween a frame 504 and an input fiber 506 receives a drive voltage at afrequency twice that of the horizontal scan frequency. Responsive to thedrive voltage, the piezoelectric driver 502 deforms. Because the fiber506 is bonded to the piezoelectric driver 502, deformation of thepiezoelectric driver 502 produces corresponding shifting of the fiber506 as indicated by the arrow 508 and shadowed fiber 510. One skilled inthe art will recognize that, depending upon the characteristics of theoptical system 500, the piezoelectric driver 502 may produce lateraltranslation of the fiber 506 or angular shifting of the fiber 506output. The optical system 500 then translates movement of the fiberoutput into movement of the perceived pixel location as in thepreviously described embodiments. While the embodiment of FIG. 23translates a fiber, the invention is not so limited. For example someapplications may incorporate translation of other sources, such as LEDsor laser diodes, may translate the position of the lens 50, or maytranslate or rotate an entire scanner, such as a biaxial MEMs scanner.

[0112] Although the embodiment of FIG. 23 shifts the input beam byshifting the position of the input fiber, other methods of shifting theinput beam may be within the scope of the invention. For example, asshown in FIG. 24, an electro-optic crystal 300 shifts the input beam 83in response to an electrical signal. In this embodiment, the beam 83enters a first face 302 of a trapezoidally shaped electro-optic crystal300, where refraction causes a shift in the direction of propagation.When the beam 83 exits through a second face 304, refraction produces asecond shift in the direction of propagation. At each face, the amountof changes in the direction or propagation will depend upon differencein index of refraction between the air and the crystal 300.

[0113] As is known, the index of refraction of electro-optical crystalsis dependent upon the electric field through the crystal. A voltageapplied across the crystal 300 through a pair of electrodes 306 cancontrol the index of refraction of the crystal 300. Thus, the appliedvoltage can control the index of refraction of the crystal 300. Thus theapplied voltage can control the angular shift of the beam 83 as itenters and exits the crystal 300 as indicated by the broken line 83 a.The amount of shift will correspond to the applied voltage. Accordingly,the amount of shift can be controlled by controlling the voltage appliedto the electrodes 306. The crystal 300 thus provides a voltagecontrolled beam shifter that can offset raster pinch.

[0114] Although the embodiments described herein have been displays,other devices or methods may be within the scope of the invention. Forexample, as shown in FIG. 24, an imager 600 includes a biaxial scanner602 and correction scanner 604 that are very similar to the scanners152, 154 of FIG. 21. The imager 600 is an image collecting device thatmay be the input element of a digital camera, bar code reader, twodimensional symbol reader, document scanner, or other image acquisitiondevice. To allow the imager 600 to gather light efficiently, the imager600 includes gathering optics 606 that collect and transmit light from atarget object 608 outside of the imager 600 onto the correction scanner604. The gathering optics 606 are configured to have a depth of field,focal length, field of view and other optical characteristicsappropriate for the particular application. For example, where theimager 600 is a two dimensional symbology reader, the gathering optics606 may be optimized for red or infrared light and the focal length maybe on the order of 10-50 cm. For reading symbols at a greater distance,the focusing optics may have longer focusing distance or may have avariable focus. The optics may be positioned at other locations alongthe optical path to allow smaller, cheaper components to be used.

[0115] The correction scanner 604 redirects light received from thegathering optics 606 as described above for the display embodiments, sothat the gathered light has a correction component before it reaches thebiaxial scanner 602. The biaxial scanner 602 scans through a generallyraster pattern to collect light arriving at the gathering optics 606from a range of angles and to redirect the light onto a group ofstationary photodetectors 610, each positioned at a respective locationand orientation, such that it images a respective “tile” of the imagefield.

[0116] Movement of the biaxial scanner 602 thus translates to imagingsuccessive points of the target object 608 onto the photodetectors 610.The photodetectors 610 convert light energy from the scanner 602 intoelectrical signals that are received by decoding electronics 612. Wherethe imager 600 is a symbology reader, the decoding electronics 612 mayinclude symbol decoding and storing circuitry and further electronicsfor assembling the image form the stored files. Where the imager is aportion of a camera, the decoding electronics 612 may includedigital-to-analog converters, memory devices and associated electronicsfor storing a digital representation of the scanned tile and furtherelectronics for assembling the image from the stored files. One skilledin the art will recognize that, although the correction scanner 604 ispositioned before the bi-axial scanner 602, it may be desirable toposition the correction scanner 604 following the biaxial scanner 602 insome applications.

[0117] Another feature of the imager 600 shown in FIG. 24 is a set ofillumination sources 614 that provide light for illuminating respectivelocations on a target object. The illumination sources 614 arepreferably of different wavelengths to ease differentiation of beams,although in some applications common wavelength devices may be used. Inone example of a multiwavelength structure where imager 600 is a symbolreader, the illumination sources 614 may include infrared or red lightemitters that emit beams of light into a beam splitter 616. The beamsplitter 616 directs the illuminating light beams into the biaxialscanner 602 where the illuminating light is redirected to the correctionscanner 604. Because the illuminating light beams are collinear with thepaths of light from the target object 608, the illuminating light beamsstrike the target object 608 at the same locations that are imaged bythe photodetectors 610. The illuminating light beams are reflected bythe target object 608 in pattern corresponding to the reflectivity ofthe respective regions of the target object 608. The reflectedilluminating light travels to the photodetectors 610 to image therespective regions light that can be used only by the photodetectors 610to image the respective regions of the target object 608. For highresolution, the area illuminated by the sources 614 or imaged by thephotodetectors 610 may be made small through a variety of known opticaltechniques. One skilled in the art will recognize that, although FIG. 24shows the correction scanner 604 positioned after the horizontal scanner602, it will often be preferable to position the correction scanner 604between the beam splitter 616 and the horizontal scanner 602. Thisallows for the mirror of the correction scanner 604 to be made small.

[0118] Alternatively, the photodetectors 610 may be mounted externallyto the scanners 602, 604 and oriented to capture light directly fromtheir respective tiles. Because each photodetector 610 is wavelengthmatched to its respective source and because the photodetectors 610 arealigned to spatially distinct regions, crosstalk between signals fromthe respective tiles may be adequately suppressed.

[0119] In one application of the imager 600 of FIG. 24, one or more ofthe illumination sources 614 includes a visible, directly modulatedlight source, such as a red laser diode or a visible wavelength lightemitted diode (LED). As shown in FIG. 25, the visible illuminationsource 614 can thus produce a visible image for the user. In theexemplary embodiment of FIG. 25, the imager can operate as a symbologyscanner to identify information contained in a symbol on the targetobject 608. Once the decoding electronics 612 identifies a desired imageto be viewed, such as an item price and identity, the decodingelectronics 612 modulates the drive current of the illumination sources614 to modulate the intensity of the emitted light according to thedesired image. When the user directs the imager 600 toward a screen 619(or the target object), the illuminating light is scanned onto thescreen 619 as described above. Because the illuminating light ismodulated according to the desired image, the visible light reflectedfrom the screen 619 is spatially modulated according to the desiredimage. The imager 600 thus acts as an image projector in addition toacquiring image data. In addition to, or as an alternative to,modulating the diode to produce an image, the diodes corresponding toeach of the regions of the target object 608 may also output continuousor pulsed beams of light that fill the entire field of view of theimager 600. The imager 600 thus provides a spotter frame 618 thatindicates the field of view to the user. Similarly, the illuminationsources 614 can be modified to outline the field of view or to produceother indicia of the field of view, such as cross hatching or fiducials,to aid the user in aligning the imager 600 to the target object 608.

[0120] In addition to compensating for raster pinch, one embodiment ofthe scanning system, shown in FIG. 28, also addresses effects of thenonlinearity of resonant and other nonlinear scanning systems. Oneskilled in the art will recognize that, although this correction isdescribed for a single light source or single detector system, theapproaches described herein are applicable to systems using more thanone light source, as presented in FIG. 10 above. For example, in oneapplication, the corrected output clock signal described below withreference to FIG. 28, drives all of the buffers 1306A-D (FIG. 13) tooutput data in parallel from buffers 1306A-D.

[0121] As shown by broken line in FIG. 26, the timing of incoming datais premised upon a linear scan rate. That is, for equally spacedsubsequent locations in a line, the data arrive at constant intervals. Aresonant scanner, however, has a scan rate that varies sinusoidally, asindicated by the solid line in FIG. 26. For a start of line beginning attime t₀ (note that the actual start of scan for a sinusoidal scan wouldlikely be delayed slightly as described above with respect to FIG. 26),the sinusoidal scan initially lags the linear scan. Thus, if the imagedata for position P₁ arrive at time t_(1A), the sinusoidal scan willplace the pixel at position P₂.

[0122] To place the pixel correctly, the system of FIG. 28 delays theimage data until time t_(1B), as will now be described. As shown in FIG.28, arriving image data V_(IM) are clocked into a line or frame buffer2200 by a counter circuit 2202 in response to a horizontalsynchronization component of the image data signal. The counter circuit2202 is a conventional type circuit, and provides an input clock signalhaving equally spaced pulses to clock the data into the buffer 2200. Inthe multisource system of FIG. 13, the four buffers 1306A-D, anddemultiplexer 1304 replace the frame buffer and the image data areclocked sequentially through the demultiplexer 1304 into the fourbuffers 1306A-D, rather than being clocked into a single frame buffer orline buffer.

[0123] A feedback circuit 2204 controls timing of output from the buffer2200 (or buffers 1306A-D of FIG. 13). The feedback circuit 2204 receivesa sinusoidal or other sense signal from the scanning assembly 82 anddivides the period of the sense signal with a high-speed second counter2206. A logic circuit 2208 produces an output clock signal in responseto the counter output.

[0124] Unlike the input clock signal, however, pulses of the outputclock signal are not equally spaced. Instead, the pulse timing isdetermined analytically by comparing the timing of the linear signal ofFIG. 26 to the sinusoidal signal. For example, for a pixel to be locatedat position P₁, the logic circuit 2208 provides an output pulse at timet_(1B), rather than time t_(1A), as would be the case for a linear scanrate.

[0125] The logic circuit 2208 identifies the count corresponding to apixel location by accessing a look-up table in a memory 2210. Data inthe look-up table 2210 are defined by dividing the scanning systemperiod into many counts and identifying the count corresponding to theproper pixel location. FIG. 27 shows this evaluation graphically for a35-pixel line. One skilled in the art will recognize that this exampleis simplified for clarity of presentation. A typical line may includehundreds or even thousands of pixels. As can be seen, the pixels will bespaced undesirably close together at the edges of the field of view andundesirably far apart at the center of the field of view. Consequently,the image will be compressed near the edges of the field of view andexpanded near the middle, thereby forming a distorted image.

[0126] As shown by the upper line, pixel location varies nonlinearly forpixel counts equally spaced in time. Accordingly, the desired locationsof each of the pixels, shown by the upper line, actually correspond tononlinearly spaced counts. For example, the first pixel in the upper andlower lines arrives at the zero count and should be located in the zerocount location. The second pixel arrives at the 100 count, but, shouldbe positioned at the 540 count location. Similarly, the third pixelarrives at count 200 and is output at count 720. One skilled in the artwill recognize that the figure is merely representative of the actualcalculation and timing. For example, some output counts will be higherthan their corresponding input counts and some counts will be lower. Ofcourse, a pixel will not actually be output before its correspondingdata arrives. To address this condition, the system of FIG. 28 actuallyimposes a latency on the output of data, in a similar fashion tosynchronous memory devices. For the example of FIG. 27, a single linelatency (3400 count latency) would be ample. With such a latency, thefirst output pixel would occur at count 3400 and the second would occurat count 3940.

[0127]FIG. 29 shows an alternative approach to placing the pixels in thedesired locations. This embodiment produces a corrected clock from apattern generator rather than a counter to control clocking of outputdata. A synch signal stripper 2500 strips the horizontal synchronizationsignal form an arriving image signal V_(IM). Responsive to the synchsignal, a phase locked loop 2502 produces a series of clock pulses thatare locked to the synch signal. An A/D converter 2504, driven by theclock pulses, samples the video portion of the image signal to producesampled input data. The sampling rate will depend upon the requiredresolution of the system. In the preferred embodiment, the sampling rateis approximately 40 Mhz. A programmable gate array 2506 conditions thedata from the A/D converter 2504 to produce a set of image data that arestored in a buffer 2508. One skilled in the art will recognize that, foreach horizontal synch signal, the buffer will receive one line of imagedata. For a 1480×1024 pixel display, the system will sample and store1480 sets of image data during a single period of the video signal.

[0128] Once each line of data is stored in the buffer 2508, the bufferis clocked to output the data to a RAMDAC 2509 that includes a gammacorrection memory 2510 containing corrected data. Instead of using thebuffer data as a data input to the gamma correction memory 2510, thebuffer data is used to produce addressing data to retrieve the correcteddata from the gamma correction memory 2510. For example, a set of imagedata corresponding to a selected image intensity I1 identifies acorresponding location in the gamma correction memory 2510. Rather thanoutput the actual image data, the gamma correction memory 2510 outputs aset of corrected data that will produce the proper light intensity atthe user's eye. The corrected data is determined analytically andempirically by characterizing the overall scanning system, including thetransmissivity of various components, the intensity versus currentresponse of the light source, diffractive and aperture effects of thecomponents and a variety of other system characteristics.

[0129] In one embodiment shown in FIG. 30 according to the invention,the data may be corrected further for temperature-versus-intensity orage-versus-intensity variations of the light source. Reference datadrives the light source while the vertical and horizontal position isout of the user's field of view. For example, at the edge of thehorizontal scan, the reference data is set to a predetermined lightintensity. A detector 2519 monitors the power out of the light source2516 and a temperature compensation circuit 2521. If the intensity ishigher than the predetermined light intensity, a gain circuit 2523scales the signal from the RAMDAC 2506 by a correction factor that isless than one. If the intensity is higher than the predetermined lightintensity, the correction factor is greater than one. While theembodiments described herein pick off a portion of the unmodulated beamor sample the beam during non-display portions of the scanning period,the invention is not so limited. For example, a portion of the modulatedbeam can be picked off during the display portion of the scanning periodor continuously. The intensity of the picked off portion of themodulated beam is then scaled and compared to the input video signal todetermine shifts in the relative intensity of the displayed light versusthe desired level of the displayed light to monitor variations.

[0130] In addition to monitoring the intensity, the system can alsocompensate for pattern dependent heating through the same correctiondata or by multiplying by a second correction factor. For example, wherethe displayed pattern includes a large area of high light intensity, thelight source temperature will increase due to the extended period ofhigh level activation. Because data corresponding to the image signal isstored in a buffer, the data is available prior to the actual activationof the light source 2516. Accordingly, the system can “look-ahead” topredict the amount of heating produced by the pattern. For example, ifthe light source will be highly activated for the 50 pixels precedingthe target pixel, the system can predict an approximate patterndependent heat effect. The correction factor can then be calculatedbased upon the predicted pattern dependent heating. Although thecorrection has been described herein for the intensity generally, thecorrection in many embodiments can be applied independently for red,green and blue wavelengths to compensate for different responses of theemitters and for variations in pattern colors. Compensating for eachwavelength independently can help limit color imbalance due to differingvariations in the signal to intensity responses of the light emitters.

[0131] Returning to FIG. 29, the corrected data output from the gammacorrection memory 2510 (as it may be modified for intensity variations)drives a signal shaping circuit 2514 that amplifies and processes thecorrected analog signal to produce an input signal to a light source2516. In response, the light source 2516 outputs light modulatedaccording to the corrected data from the gamma correction memory 2510.The modulated light enters a scanner 2518 to produce scanned, modulatedlight for viewing.

[0132] The clock signal that drives the buffer 2508, correction memory2510, and D/A converter 2512 comes from a corrected clock circuit 2520that includes a clock generator 2522, pattern memory 2524 and risingedge detector 2526. The clock generator 2522 includes a phase lockedloop (PLL) that is locked to a sense signal from the scanner 2518. ThePLL generates a high frequency clock signal at about 80 MHz that islocked to the sense signal. The high frequency clock signal clocks datasequentially from addresses in the pattern memory 2524.

[0133] The rising edge detector 2526 outputs a pulse in response to each0-to-1 transition of the data retrieved from the pattern memory 2524.The pulses then form the clock signal CKOUT that drives the bufferoutput, gamma correction memory 2510, and D/A converter 2512.

[0134] One skilled in the art will recognize that the timing of pulsesoutput from the edge detector 2526 will depend upon the data stored inthe pattern memory 2524 and upon the scanning frequency f_(SCAN) of thescanner 2518. FIG. 31 shows a simplified example of the concept. Oneskilled in the art will recognize that, in FIG. 31, the data structureis simplified and addressing and other circuitry have also been omittedfor clarity of presentation.

[0135] In the example of FIG. 31, if the scanning frequency f_(SCAN) is20 kHz and clock generator 2522 outputs a clock signal at 4000 times thescanning frequency f_(SCAN), the pattern memory 2524 is clocked at 80MHz. If all bits in an addressed memory location 2524A are 0, notransitions of the output clock occur for 16 transitions of thegenerator clock. For the data structure of location 2524B, a singletransition of the output clock occurs for 16 transitions of thegenerator clock. Similarly, location 2524C provides two pulses of thegenerator clock in one period of the scan signal and location 2524Eprovides eight pulses of the generator clock in one period.

[0136] The number and relative timing of the pulses is thus controlledby the data stored in the pattern memory 2524. The frequency of thegenerator clock on the other hand depends upon the scanner frequency. Asthe scanner frequency varies, the timing of the pulses thus will vary,yet will depend upon the stored data in the pattern memory.

[0137] The approaches of FIGS. 29 and 30 are not limited to sinusoidalrate variation correction. The clock pattern memory 2524 can beprogrammed to address many other kinds of nonlinear effects, such asoptical distortion, secondary harmonics, and response timeidiosyncrasies of the electronic and optical source.

[0138] Moreover, the basic structure of FIG. 29 can be modified easilyto adapt for vertical scanning errors or optical distortion, byinserting a bit counter 2530, look up table 2532, and verticalincrementing circuit 2534 before the buffer 2508, as shown in FIG. 30.The counter 2530 addresses the look up table 2532 in response to eachpulse of the input clock to retrieve two bits of stored data. Theretrieved data indicate whether the vertical address should beincremented, decremented or left unaffected. The data in the look uptable 2532 is determined empirically by measuring optical distortion ofthe scanning system and optics or is determined analytically throughmodeling. If the address is to be incremented or decremented, theincrementing circuit increments or decrements the address in the buffer2508, so that data that was to be stored in a nominal memory locationare actually stored in an alternate location that is one row higher orlower than the nominal location.

[0139] A graphical representation of one such data structure is shown inthe simplified example FIG. 32. In this example, the first three sets ofdata bits 3202 for the first line of data (line 0) are stored in thefirst memory row, the next three sets of data bits 3204 for the firstline are stored in the second memory row, and the last three sets ofdata bits are stored in the third memory row. One skilled in the artwill recognize that this example has been greatly simplified for clarityof presentation. An actual implementation would include many more setsof data and may utilize decrementing of the row number as well asincrementing.

[0140] The result is that some portion of the data for one line is movedto a new line. The resulting data map in the buffer 2508 is thusdistorted as can be seen from FIG. 32. However, distortion of the datamap can be selected to offset vertical distortion of the image caused byscanning and optical distortion. The result is that the overall systemdistortion is reduced. Although the embodiment of FIG. 30 showscorrection of vertical distortion by adjusting the position of datastored in the buffer 2508, other approaches to this correction may beimplemented. For example, rather than adjusting the addresses of thestorage locations, the addresses used for retrieving data from thebuffer 2508 to the RAMDAC 2509 can be modified.

[0141] As noted above, in many applications, it is desirable to controlthe scanning frequencies of one or more scanners. In non-resonant or lowQ applications, simply varying the frequency of the driving signal canvary the scanning frequency. However, in high Q resonant applications,the amplitude response of the scanners may drop off dramatically if thedriving signal differs from the resonant frequency of the scanner.Varying the amplitude of the driving signal can compensate somewhat, butthe magnitude of the driving signal may become unacceptably high in manycases. Consequently, it is undesirable in many applications to try tocontrol the scanner frequency f_(SCAN) simply by controlling the drivingsignal frequency and/or amplitude.

[0142] One approach to controlling the frequency f_(SCAN) is shown inFIGS. 33 and 34 for a MEMs scanner 3300. The scanner 3300 includes fourtuning tabs 3302A-D positioned at corners of a mirror body 3304. Thetuning tabs 3302A-D are flexible projections that are integral to themirror body 3304. Fixed rigid projections 3305 project from the mirrorbody 3304 adjacent to the tuning tabs 3302A-D, leaving a small gaptherebetween.

[0143] Each of the tuning tabs 3302A-D carries a ground electrode 3306coupled by a conductor 3310 to an external electrode 3312 to form anelectrical reference plane adjacent to the respective tab 3302A-D. Eachof the rigid projections 3306A-D carries a respective hot electrode 3308controlled by a respective external electrode 3316A-D, that allowscontrol of the voltage difference between each tuning tab 3302A-D andits corresponding rigid projection 3306A-D.

[0144] Each flexible tab 3302A-D is dimensioned so that it bends inresponse to an applied voltage difference between the tab 3302A-D andthe adjacent rigid projection 3306, as shown in FIG. 34. The amount ofbending will depend upon the applied voltage, thereby allowingelectrical control of tuning tab bending.

[0145] One skilled in the art will recognize that the resonant frequencyof the scanner 3300 will be a function of the mass of the mirror 3304,the dimensions and mechanical properties of torsion arms 3317 supportingthe mirror 3304, and the locations 3318 of the centers of mass of eachhalf of the mirror 3304 (including its tabs 3302A-D and rigidprojections 3306) relative to the axis of rotation of the mirror 3304.Bending the flexible tabs shifts the centers or mass slightly inwardlyfrom the original locations 3318 to new locations 3320. Because thecenters of mass are located closer to the axis of rotation, the scanningfrequency increases slightly. Increasing the voltage on the fixedprojections 3306 thus can increase the resonant frequency of the scanner3300.

[0146] The use of electronically controlled elements to controlresonance in a scanner is not limited to controlling the horizontalscanning frequency. For example, in the embodiment of FIG. 35, a mirrorbody 3500 has interdigitated comb drives 3502 that extend from thebody's edges. Comb driven actuators are known structures, beingdescribed for example in Tang, et al., ELECTROSTATIC-COMB DRIVE OFLATERAL POLYSILICON RESONATORS, Transducers '89, Proceedings of the5^(th) International Conference on Solid State Sensors and Actuators andEurosensors III, Vol. 2, pp. 328-331, June 1990, which is incorporatedherein by reference.

[0147] Respective conductors 3504 extend from each of the comb drives3502 to allow tuning voltages Vtunel, Vtune2 to control the comb drives3502. As is known, applied voltages produces lateral forces F1, F2 inthe comb drives 3502. Flexible arms 3506 at the distal ends of the combdrives 3502 bend in response to the forces F1, F2, thereby shifting themass of the flexible arms 3506 relative to the center of mass 3508 ofthe respective half of the mirror body. Because the position shift isparallel to the axis of rotation of the mirror body 3500, the horizontalresonant frequency does not shift significantly. However, if thevoltages are set such that the flexible arms experience differentposition shifts, the mirror body 3500 can be made slightly unbalanced.The mirror body 3500 will then begin to approximate the Lissajouspattern of FIG. 20. Adjusting the tuning voltages Vtune1, Vtune2produces a corresponding adjustment in the scan pattern. Where themasses of the flexible portions 3506 and the voltages Vtune1, Vtune2 arechosen appropriately, the resonant frequency of vibrations from theunbalanced mirror body will be an integral multiple of the horizontalscanning frequency and the Lissajous pattern will be stable. Bymonitoring the scan pattern and adjusting the tuning voltages Vtune1,Vtune2 accordingly, the Lissajous pattern can be kept stable. Thus, theelectronically controlled structures can assist in pinch correction. Ifit is undesirable in a particular application to unbalance the mirrorbody 3500, other approaches, such as a piezoelectrically pivoted mirrorcarried by mirror body 3500 may provide the desired orthogonal componentto produce the “bow tie” effect.

[0148]FIG. 36 shows an alternative approach to controlling the resonantfrequency of a scanner 3600. In this embodiment, the scanner 3600 ishoused on a platform 3602 in a sealed package 3604 having a transparentlid 3606. The package 3604 also contains a gas, such as a helium orargon mix, at a low pressure. The resonant frequency of the scanner 3600will depend, in part, upon the pressure of within the package 3602 andthe properties of the gas, as is described in Baltes et. al., THEELECTRONIC NOSE IN LILLIPUT, IEEE Spectrum, September 1998, pp. 35-39,which is incorporated here by reference. Unlike conventional sealedpackages, the package 3602 includes a pair of outgassing nodules 3610concealed beneath the platform 3602.

[0149] The nodules 3610 are formed from an outgassing material, such asisopropanol in a polymer, atop a resistive heater 3611. Electricalcurrent causes resistive heating of the heater 3611, which, in turncauses the nodule 3610 to outgas. An electronic frequency controller3614, controls the amount of outgassing by applying a controlled currentthrough pairs of electrodes 3612 positioned on opposite sides of each ofthe nodules 3610. The increased gas concentration reduces the resonantfrequency of the scanner 3600. For greater frequency variation,absorptive polymer segments 3618 coat the scanners torsion arms 3620 to“amplify” the absorptive effect on resonant frequency. Alternatively,the frequency may be controlled by a gettering technique. In such anapproach, an absorptive material is placed in the package 3602. Theabsorptive material may be heated, cooled, or otherwise activated tocontrol it's absorption of a selected gas, thereby reducing the pressurewithin the package 3602 to vary the frequency.

[0150] Typically, the above-described variable or “active” tuningapproaches are most desirable for producing small frequency variations.For example, such small frequency adjustments can compensate forresonant frequency drift due to environmental effects, aging, orinternal heat buildup. To reduce the difficulty of active tuningapproaches or to eliminate active tuning entirely, it is desirable inmany applications to “tune” the resonant frequency of a scanner tominimize the difference between the scanner's uncompensated resonantfrequency and the desired scan frequency. Such frequency differences maybe caused by processing variations, material property variations, orseveral other effects.

[0151]FIG. 37 shows one approach to tuning the scanner's uncompensatedresonant frequency, in which a scanner 3700 is fabricated with integraltuning tabs 3702A-B, 3704A-B, 3706A-B, 3708A-B, 3710, and 3712.Initially, the scanner's mirror body 3714 and torsional arms 3716 aredimensioned to produce a resonant frequency (with all of the tuning tabs3702A-B, 3704A-B, 3706A-B, 3708A-B, 3710, and 3712 attached) that isslightly below the desired resonant frequency. Once the scanner 3700 isassembled, the resonant frequency can be measured in a variety offashions. For example, the scanner 3700 can be driven in one of thetechniques described previously and the mirror response can be monitoredoptically. Alternatively, impedance versus frequency measurements mayalso provide the resonant frequency relatively quickly.

[0152] The determined resonant frequency is then compared to the desiredresonant frequency to identify a desired frequency compensation. Basedupon the identified frequency compensation some of the tuning tabs3702A-B, 3704A-B, 3706A-B, 3708A-B, 3710, and 3712 can be removed, forexample by laser trimming or mechanical force to reduce the mass of themirror body 3714. As is known, lowering the mass of the mirror body 3714(in the absence of other variations) will increase the resonantfrequency. The number and position of the tabs to be removed for theidentified frequency compensation can be determined through modeling orempirical data. Preferably, the removed tuning tabs are positionedsymmetrically relative to the center of mass of the respective half ofthe mirror body and with respect to the axis of rotation of the mirrorbody 3714. To make this symmetricity easier, the tuning tabs 3702A-B,3704A-B, 3706A-B, 3708A-B, 3710, and 3712 are positioned in thesymmetric locations about the mirror body 3714. For example, tuning tabs3702A-B and 3704A-B form a quartet of tabs that would typically beremoved as a group. Similarly, tuning tabs 3710 and 3712 form a pair oftabs that would typically be removed as a pair.

[0153] While the tuning tabs 3702A-B, 3704A-B, 3706A-B, 3708A-B, 3710,and 3712 in FIG. 37 are shown as equally sized for ease of presentation,it is not always necessary or even desirable to make them the same size.In some applications, such tabs may be variably sized to allow greaterflexibility in tuning.

[0154] As described above with respect to FIG. 12, tiling in twodimensions can allow a large, high resolution display with less demandupon a scanner. FIG. 38 shows one difficulty that may arise when fourseparate sources 3800, 3802, 3804, 3806 feed a common scanner 3808. Ascan be seen from the ray tracing for the lower left scanner 3800, theupper right source 3804 is positioned within an expected scanning field3810 of the lower left source 3800. With no further adjustment, theupper right source 3804 would be expected to occlude a portion of theimage from the lower left source 3800, producing an unilluminated regionin the corresponding tile.

[0155]FIG. 39 shows one approach in which the effects of overlapping ofsources and beams can be reduced. In this embodiment, light arrivesthrough separate fibers 3900, 3902, 3904, 3906 and is gathered andfocused by respective GRIN lenses 3908, 3910, 3912, 3914 onto respectiveturning mirrors 3916, 3918, 3920, 3922. As is visible for two of themirrors 3916, 3922 in FIG. 40, the turning mirrors 3916, 3922 are verysmall mirrors that redirect light from their respective GRIN lenses3908, 3914 toward a curved, partially reflective mirror 3924. The mirror3924 returns the incident light toward a centrally positioned scanner3926 that scans periodically, as described previously. The scanned lightpasses through the partially transmissive mirror 3924 toward an imagefield 3928 where an image can be viewed.

[0156] As can be seen in FIG. 40, the GRIN lenses 3908, 3914 gatherdiverging light from the respective fibers 3900, 3906 and reduce thebeam width to substantially its minimum diameter at the respectiveturning mirror 3916, 3922. The beam 3930 then expands as it travels tothe curved mirror 3924. The curved mirror 3924 converts the expandingbeam 3930 into a substantially collimated or slightly converging beam3932 having a diameter slightly smaller than the mirror width W of thescanner 3926.

[0157] It can be seen in FIG. 40 that the turning mirrors 3916, 3918,3920, 3922 will block light from other turning mirrors during a portionof their scans. However, because the turning mirrors block only smallsection of the beams and because the beams converge at the image field3924, the effect will be a slight dimming of the corresponding pixel.Uncompensated, this might produce a slight variation from the desiredpixel intensity. However, the programmable gate array 2506 describedabove with respect to FIG. 29 can pre-weight the intensity to offset thedimming effects of the turning mirrors 3916, 3918, 3920, 3922.

[0158] To further improve efficiency the display of FIGS. 39 and 40 canalso take advantage of properties of polarized light. In someapplications, the fibers 3900, 3902, 3904, 3906 (or other light sourcessuch as laser diodes) emit polarized light. A polarization dependentreflector 3934, such as 3M's Dual Brightness Enhancement Film coats theinner surface of the mirror and reflects the polarized incident beam3930. As the reflected beam 3932 travels to the scanner 3926, the beam3932 passes through a quarter wave plate that converts beam to acirculatory polarized beam. The beam 3932 is then reflected by thescanner 3926 and passes through the quarter wave plate once again,converts the circulatory polarized beam back to a linearly polarizedbeam, now orthogonal to the original beam 3930. The orthogonallypolarized beam passes efficiently through the polarization dependentreflector 3934 and travels to the image field 3928.

[0159]FIG. 41 shows how the use of a tiling approach can reduce rasterpinch without a correction scanner. In this embodiment, modulated lightfrom an input fiber 4102 enters one or the other of a pair oftransmission fibers 4104, 4106 as dictated by an optical switch 4108.Light exits the transmission fibers 4104, 4106 and strikes a commonscanner 4110 that scans light from the first fiber 4104 onto a firstregion 4112 of an image field 4114 and scans light from the second fiber4106 onto a second region 4116 of the image field 4114. The fibers 4104,4106 are oriented so that the first and second regions 4112, 4116overlap very slightly in an overlap area 4118.

[0160] During forward sweeps of the scanner 4110, an electroniccontroller 4120 activates the switch 4108 so that light passes throughthe second fiber 4106. The scanner 4110 thus redirects the light along afirst scan line 4122 in the second region 4116. At the end of theforward sweep, the controller 4120 activates the switch 4108 so thatlight now passes through the first fiber 4104 and is scanned along afirst scan line 4124 in the first region 4112. For each subsequent sweepof the scanner 4110, the controller 4120 activates the switch to producesets of lines in each of the regions 4112, 4116. Because the verticalscan continues during the forward sweeps, the lines may be slightlytilted, as shown in FIG. 41. While such tilt is typically not observableby a viewer, if desired, custom optics can produce a “counter”-tilt thatoffsets the scanning tilt. Alternatively, the image data may bepredistorted by the programmable gate array 2506 described above withrespect to FIG. 29 to compensate.

[0161] This structure is not limited to two horizontal tiles or to asingle light emitter. For example, as shown in FIG. 42, light from twofibers can be switched into four fibers to produce a 2-by-2 tiled image.

[0162] In this approach, an input fiber 4200 is coupled to four fibers4202, 4204, 4206, 4208 by a set of optical switches 4210, 4212, 4214,where each fiber feeds a scanning assembly 4216 from a respective angle.A switch controller 4220 activates the switches 4210, 4212, 4214according to the direction of the sweep and according to the trackedlocation of the user's gaze, as provided by a gaze tracker (not shown).The gaze tracker may be any known apparatus for determining gazedirection.

[0163] For example, when the user looks at the top half of the image, afirst fiber 4206, aligned to produce an image in the upper left tile4222 feeds the scanning assembly 4216 during the forward sweeps. Asecond fiber 4208, aligned to produce an upper right tile 4224 feeds thescanning assembly 4216 during reverse sweeps. When the user looks at thelower half of the image, a third fiber 4204, aligned to produce thelower left tile 4226, feeds scanning assembly 4216 during forwardsweeps. A fourth fiber 4202, aligned to produce the lower right tile4228, feeds the scanning assembly 4216 during reverse sweeps. While eachof the fibers 4200, 4206, 4208, 4204 is represented as a single fiber,in some applications each fiber 4200, 4206, 4208, 4204 may actuallyinclude a plurality of fibers 4200, 4206, 4208, 4204. In suchapplications each fiber 4200, 4206, 4208, 4204 is fed by a plurality ofinput fibers 4200 and a corresponding plurality of switch sets. Such anembodiment advantageously allows a plurality of lines to be writtensimultaneously. Writing a plurality of lines simultaneously reduces thefrequency of the horizontal scanner relative to the single line writingapproaches described above, thereby reducing the difficulty of scanning.Also, providing light simultaneously from a plurality of light emittersreduces the amount of light energy required from each source for a givendisplay brightness and reduces the modulation frequency of the beam.This reduces the performance requirements of the light sources, therebydecreasing the cost and complexity of the overall display.

[0164] While the embodiments of FIGS. 41 and 42 have been describedherein using fibers and optical switches, in some applications, discretelight sources, such as laser diodes, LEDs, microlasers, or gas lasersmay replace each fiber. In such applications, electrical switches (e.g.,transistors) selectively control drive currents to the respectivesources or control external modulators aligned with the respectivesources to control feeding of light during forward an reverse sweeps ofthe mirror.

[0165] Although the invention has been described herein by way ofexemplary embodiments, variations in the structures and methodsdescribed herein may be made without departing from the spirit and scopeof the invention. For example, the positioning of the various componentsmay also be varied. In one example of repositioning, the correctionscanners can be positioned in the optical path either before or afterthe other scanners. Also, an exit pupil expander may be added or omittedin many applications. In such embodiments, conventional eye tracking maybe added to ease coupling of the scanned beam to the eye. Moreover, thescanning system can be used for projection displays, optical storage anda variety of other scanned light beam applications, in addition toscanned retinal displays. Further, a variety of other timing controlmechanisms, such as programmable delays, may be used to compensate forthe variable speed of the scanner in place of the approaches describedwith reference to FIGS. 24-31. Additionally, in some applications it maybe desirable for ease of positioning or for other reasons to use aplurality of scanners, each of which may be fed by one or more beams. Insuch a structure, each scanner and its corresponding light sourcesproduce respective sets of tiles. The overall image is than formed bycombining the sets of tiles from each of the scanners, either byadjacent positioning or by overlapping. Although overlapping isgenerally preferred only where each scanner is used for a respectivewavelength, in some applications overlapping may be used for interlacingor other approaches to image combination.

[0166] In another alternative approach to timing and distortioncorrection, the memory map may be undistorted and addressed at aconstant rate. To compensate for nonlinearity of the scanner, the datafor each location is derived from the retrieved image data and output atfixed increments. Referring to FIG. 27, for example, data would beoutput at a time 1500, even though this time did not correspond directlyto a pixel time. To compensate, the buffer 2508 is addressed at the10^(th) and 11^(th) locations for this line. Then, the output data is aweighted average of the data from the 10_(th) and 11_(th) locations.Thus, the buffer 2508 is clocked at a constant rate and pixels areoutput at a constant rate. Yet, by controlling the addressing circuitrycarefully and performing a weighted averaging, the output data issinusoidally corrected. Also, although the light emitters and lightsources described herein utilize laser diodes or LEDs, with or withoutfibers, a variety of other light emitters such as microlasers, gaslasers, or other light emitting devices may desirable in someapplications. Moreover, although the exemplary scanning assembliesdescribed herein utilize torsionally mounted mirrors, other scanningassembly structures, such as spinning polygons, comb drive mirrors,acousto optic scanners, and other scanning structures may be within thescope of the invention. Also, while the beams are shown as convergingupon a single scanner, in some applications it may be desirable to useseparate scanners for each beam of light or to use a plurality ofscanners that each reflect a plurality of beams. Accordingly, theinvention is not limited except as by the appended claims.

What is claimed is:
 1. A device for displaying an image having first andsecond image portions in response to an image signal, comprising: ascanning mirror mounted for periodic motion through a selected scanningangle; a driver coupled to the scanning mirror and operative to sweepthe scanning mirror through the selected scan pattern; a first lightsource responsive to a first input signal to emit a first modulatedlight beam along a first light path relative to the first light source,the first light source being oriented such that the first light pathstrikes the scanning mirror at a first angle at which the scanningmirror reflects the first modulated light beam to a first image regionin which the first modulated light beam defines the first image portion;a second light source responsive to a second input signal to emit asecond modulated light beam along a second light path relative to thesecond light source, the second light source being oriented such thatthe second light path strikes the scanning mirror at a second angle atwhich the scanning mirror reflects the second modulated light beamtoward a second image region in which the second modulated light beamdefines the second image portion, and wherein the orientation of thefirst and second light sources is selected such that the first andsecond regions are substantially non-overlapping, and electronic controlcircuitry having an input port for receiving the input signal and firstand second outputs coupled to the first and second light sourcesrespectively, the control circuitry being responsive to the image signalto produce the first and second input signals.
 2. The device of claim 1wherein the first light source includes: a light emitter; and amodulator external to the light emitter.
 3. The device of claim 1wherein the first light source includes a directly modulatable lightemitter.
 4. The device of claim 1 wherein the first light sourceincludes: a light emitter; and an optical fiber, optically coupled tothe light emitter.
 5. The device of claim 1 wherein the electroniccontrol circuitry includes a clock circuit responsive to the imagesignal to produce a nonuniform clock signal having a nonuniformitycorresponding to an expected nonlinearity of the scanning mirror scanpattern.
 6. The device of claim 1 wherein the scanning mirror istorsionally mounted to a base.
 7. The device of claim 1 furtherincluding a single light emitter that provides light for both of thefirst and second light sources.
 8. The device of claim 1 wherein each ofthe first and second light sources includes a separate light emitter. 9.The device of claim 8 wherein each of the first and second light sourcesincludes a plurality of light emitters, each at a separate wavelength.10. The device of claim 8 wherein each of the light emitters includes anLED.
 11. The device of claim 8 wherein each of the light emittersincludes a laser diode.
 12. An image display for producing an image inan image field, comprising: a first light source oriented to emit afirst beam along a first optical path; a second light source oriented toemit a second beam along a second optical path that is not parallel tothe first optical path, the second optical path converging with thefirst optical path at a convergence location; a mirror positionedsubstantially at the convergence location and oriented to direct thefirst beam to a first region of the image field and oriented to directthe second beam to a second region of the image field different from thefirst region, the mirror being mounted for pivoting about a pivot axisin a forward direction and a reverse direction; a drive mechanismcoupled to the mirror and operative to pivot the mirror about the pivotaxis in a predetermined scan pattern including movement in each of theforward and reverse directions; and an electronic controller coupled toboth of the first and second light sources, the controller beingoperative to activate the first light source when the mirror pivots inthe forward direction and to activate the second light source when themirror pivots in the reverse direction.
 13. The image display of claim12 wherein the predetermined scan pattern is biaxial.
 14. The imagedisplay of claim 12 wherein each of the first and second light sourcesincludes a respective laser.
 15. The image display of claim 12 whereineach of the first and second light sources includes a plurality of lightemitters, each light emitter providing light in a respective wavelengthrange.
 16. The image display of claim 15 wherein each of the first andsecond light sources includes red, green, and blue light emitters. 17.The image display of claim 12 further including a common light emitterand wherein the first and second light sources share the common lightemitter.
 18. The image display of claim 17 further including a switchinterposed between the common light emitter and each of the first andsecond light paths and wherein the electronic controller is coupled tothe switch and operative to activate the switch.
 19. A scanningapparatus, comprising: a central scanning mirror; a mirror actuatorcoupled to the mirror and operative to drive the mirror through ascanning pattern having a period; a first light source oriented to emita first beam of light toward the central mirror during a first portionof the period; a second light source oriented to emit a second beam oflight toward the central mirror during a second portion of the perioddifferent from the first; an electronic light source drive coupled tothe first and second light sources, the light source driver beingoperative to selectively actuate one or the other of the first andsecond light sources, wherein the first light source is oriented suchthat the first light beam illuminates a first area during the firstportion and the second light source is oriented such that the secondlight beam illuminates a first area different from the first area duringthe second portion.
 20. The scanning apparatus of claim 19 wherein thefirst and second light sources include a common light emitter.
 21. Thescanning apparatus of claim 19 wherein each of the first and secondlight sources includes a respective light emitter responsive to arespective current to produce the respective beam light.
 22. A method ofscanning a target area according to a driving signal, comprising thesteps of: emitting a first beam of light from a first location along afirst path; emitting a second beam of light from a second location alonga second path different from the first path; redirecting the first beamof light from the first path along a first periodic scan pattern to afirst region of the target area; redirecting the second beam of lightfrom the second path along a second periodic scan pattern synchronizedto the first scan pattern to a second region of the target areadifferent from the first region; modulating the first beam of light inresponse to a first portion of the driving signal; modulating the secondbeam of light in response to a second portion of the driving signal. 23.The method of claim 22 wherein the step of redirecting the first beam oflight from the first path along a first periodic scan pattern to a firstregion of the target area includes: receiving the first beam of lightwith a scanning apparatus; and scanning the first beam of light throughthe first periodic scan pattern with the scanning apparatus.
 24. Themethod of claim 23 wherein the step of redirecting the second beam oflight from the second path along a second periodic scan patternsynchronized to the first scan pattern to a second region of the targetarea different from the first region includes: receiving the second beamof light with the scanning apparatus; and scanning the second beam oflight through the second periodic scan pattern with the scanningapparatus.
 25. The method of claim 24 wherein the scanning apparatusincludes a scanning mirror and wherein scanning the first beam of lightthrough the first periodic scan pattern and scanning the second beam oflight through the second periodic scan pattern include moving the mirroralong a mirror scan path.
 26. The method of claim 25 wherein the step ofmodulating the first beam of light in response to a first portion of thedriving signal occurs during a first interval in which the mirror is ina first portion of the mirror scan path in the step of modulating thesecond beam of light in response to a second portion of the drivingsignal occurs during a second interval different from the first intervalin which the mirror is also in the first portion of the mirror scanpath.
 27. A method of correcting raster pinch in a scanned image captureor display system including a scanning apparatus, comprising the stepsof: activating the scanning apparatus for periodic scanning over aplurality of scanning periods; during a first portion of each of thescanning periods defining a first scan path between a first opticaldevice and the target area by directing light to or receiving light fromthe target area with the scanning apparatus; during a second portion ofeach of the scanning periods different from the first portion defining asecond scan path between the second optical device and the target areaby directing light to or receiving light from the target area with thescanning apparatus; and orienting the first optical device, secondoptical device and scanning apparatus such that the first and secondscan paths define immediately adjacent and substantially nonoverlappingregions in the target area.
 28. The method of claim 27 wherein the firstand second optical devices are light emitters, further includingactivating the first optical device only during the first portion ofeach of the scanning periods and activating the second optical deviceonly during the second portion of each of the scanning periods.
 29. Themethod of claim 27 wherein each of the first and second scan paths aresubstantially along the common axis.
 30. The method of claim 29 furtherincluding translating the first scan path along a second axis differentfrom the common axis during the second portions of the scanning periods.31. The method of claim 30 further including translating the second scanpath along the second axis during the first portions of the scanningperiods.
 32. A method of producing an image for viewing, comprising thesteps of: emitting a first light beam from a first location; resonantlyscanning the first light beam about a first axis at a first frequency;scanning the first light beam about a second axis different from thefirst axis at a second frequency, while continuing to scan the firstlight beam about the first axis; emitting a second light beam from asecond location different from the first location; resonantly scanningthe second light beam about the first axis at the first frequency;scanning the second light beam about the second axis at the secondfrequency, while continuing to scan the second light beam about thefirst axis; modulating the first light beam in a pattern correspondingto a first portion of the image, synchronously with the step ofresonantly scanning the first light beam about the second axis; andmodulating the second light beam in a pattern corresponding to a secondportion of the image different from the first portion, synchronouslywith the step of resonantly scanning the second light beam about thesecond axis; and wherein the steps of emitting the first light beam andemitting the second light beam are temporally nonoverlapping.
 33. Themethod of claim 32 wherein the steps of resonantly scanning the firstand second light beams about the first axis includes: oscillating aturning mirror at the first frequency; and redirecting the first andsecond light beams with the oscillated turning mirror.
 34. The method ofclaim 32 wherein the step of resonantly scanning the first and secondlight beams about the second axis includes: pivoting the turning mirrorat the second frequency; and redirecting the first and second lightbeams with the turning mirror.